Blocked isocyanate compounds are compounds produced by reacting an isocyanate group with an active hydrogen group-containing compound (a blocking agent) to render the compounds inert at room temperature. Upon heating of the blocked isocyanate compounds, the blocking agent is dissociated, and the isocyanate group is regenerated. By virtue of this property, when the isocyanate group in a curing agent is blocked, the isocyanate compounds can be previously blended with an active hydrogen group-containing main agent and thus have been extensively used in coating agents, adhesives, molding materials and the like.
Conventional blocking agents for blocking the isocyanate group include alcohols, phenols, lactams, oximes, alkyl acetoacetate, alkyl malonates, phthalimides, imidazoles, hydrogen chloride, hydrogen cyanide, and sodium hydrogensulfite. Blocked isocyanate compounds produced by blocking the isocyanate group by, among the above blocking agents, substituted phenols, oximes, alkyl acetoacetates, alkyl malonates, phthalimides, imidazoles, hydrogen chloride, hydrogen cyanide, or sodium hydrogensulfite, cause dissociation at relatively low temperatures to regenerate the isocyanate group. For example, according to patent document 1, for hexamethylene diisocyanate blocked by the following compounds, the dissociation temperature upon heating for 20 min is as follows.
n-Butanol: about 170° C.
Phenol: about 160° C.
Caprolactam: about 160° C.
Methyl ethyl ketone oxime: about 150° C.
Acetoacetic ester: about 140° C.
Diethyl malonate: about 130° C.
Regarding the blocked isocyanate compounds produced by blocking the isocyanate group with blocking agents, for example, patent document 2 exemplifies polyblocked isocyanate compounds produced using 1,2,4-triazole and 3,5-dimethylpyrazole as the blocking agent, patent document 3 exemplifies polyblocked isocyanate compounds produced using an acetoacetic ester as the blocking agent, and patent document 4 exemplifies polyblocked isocyanate compounds produced using a diester of malonic acid as the blocking agent.
Regarding the reaction for blocking the isocyanate group with a pyrazole compound, for example, patent document 5 exemplifies a method in which polyisocyanate is reacted with 3,5-dimethylpyrazole in an organic solvent at 50° C. to 110° C., preferably 70° C. to 90° C., and patent document 6 exemplifies a method in which polyisocyanate is reacted with a pyrazole compound in an organic solvent at a temperature below 50° C. In the methods described in these patent documents, since the isocyanate group is reacted with the pyrazole compound in an organic solvent, these methods suffer from a problem that the solvent used in the reaction remains in the product after the reaction and, further, the removal of the solvent is difficult. Further, it should be noted that, in these patent documents, there is no description on a reaction of the blocking agent with a polymerizable compound such as an ethylenically unsaturated group-containing isocyanate compound.
Regarding the blocking of the isocyanate group in the ethylenically unsaturated group-containing isocyanate compound with a pyrazole compound, for example, patent document 7 exemplifies a method that comprises heating pyrazole or its derivative to a temperature at or above the melting point for melting, or dissolving or dispersing pyrazole or its derivative in an inert solvent such as toluene or xylene, and adding an ethylenically unsaturated group-containing compound or a solution of the compound dissolved in an inert solvent to the melt or the solution or dispersion. This method is advantageous in that the blocked isocyanate compound can be stably produced without causing polymerization of the blocked isocyanate compound and, at the same time, substantially no by-product is produced.
In the method described in patent document 7 in which pyrazole or its derivative is heated to a temperature at or above the melting point, however, pyrazole or its derivative is sublimated. Therefore, in order to allow the reaction to proceed smoothly, the use of an excessive amount of pyrazole or its derivative is necessary. This is disadvantageous from the viewpoint of productivity. Accordingly, a production process which can produce a blocked isocyanate compound with higher efficiency has been desired. On the other hand, in the method described in patent document 7 in which an inert solvent is used, the reaction can be carried out at relatively low temperatures, and, thus, advantageously, the sublimation of pyrazole or its derivative can be prevented and the reaction can proceed smoothly. Since, however, an inert solvent such as toluene or xylene is used, this method poses a problem of safety to the human body and environment and, at the same time, suffers from a problem of a complicated production process. Further, the removal of the inert solvent used in the reaction is difficult, and the solvent remains in the product. Therefore, there is a possibility that the blocked isocyanate compound thus obtained cannot be used, for example, in starting monomers of various coating agents, adhesives and molding materials. This has led to a demand for a production process of a blocked isocyanate compound that can eliminate the need to use any inert solvent.
Patent document 1: Japanese Patent Laid-Open No. 017116/1991
Patent document 2: Japanese Patent Laid-Open No. 304843/1995
Patent document 3: Japanese Patent Laid-Open No. 116420/1977
Patent document 4: Japanese Patent Laid-Open No. 121065/1982
Patent document 5: Japanese Patent Laid-Open No. 225509/1996
Patent document 6: Japanese Patent Laid-Open No. 104726/1996
Patent document 7: Japanese Patent Laid-Open No. 316663/1998
Accordingly, an object of the present invention is to provide a production process of a blocked isocyanate compound, that is, a production process of a blocked isocyanate compound comprising blocking an isocynate group in an ethylenically unsaturated group-containing isocyanate compound with a pyrazole compound, which production process can produce the compound with higher efficiency without the need to use any inert solvent.